Method for coding a video image flux

ABSTRACT

The invention lies within the area of image compression coding, in particular of MPEG type, which uses recoding help data. The invention affords a solution for reusing the help data selectively. To this end, the invention adjoins an electronic signature representative of the constraints of the coder. Thus, it becomes possible to determine whether the help data are reusable or not for a subsequent coding using other constraints.

[0001] The invention relates to a method of coding a video image stream.More particularly, the invention relates to the repetitive coding ofvideo images with the help of a compression algorithm with informationloss.

[0002] The transmission of a digital video image stream requires aconsiderable bit rate, of the order of 270 Mb/s for images of studioquality. Now, the transmission channels which permit such bit rates arerelatively expensive for long distances. In order to be able to usetransmission channels of lower capacity, it is known to resort to imagecompression algorithms.

[0003] Among the compression algorithms, it is known to use compressionalgorithms with or without information loss. The loss less algorithmscarry out a coding based on the information redundancies of each imageor of a plurality of images. The loss less algorithms have the advantageof not degrading the quality of the signal but have the defect of havinga variable bit rate which depends on the image to be coded.

[0004] The codes with loss delete image information elements which arenot normally visible and which make it possible to reduce theinformation to be transmitted. The codes with loss make it possible toobtain a constant or limited bit rate by varying the rate of informationloss.

[0005] At present, numerous image compression algorithms withinformation loss are known. By way of non-limiting indication, thestandards ITU-T H261 and ITU-T H263 exist for videophony, and ISO/IEC11172 and ISO/IEC 13818 exist for television pictures, the latter beingknown by the name MPEG (standing for Motion Picture Expert Group).

[0006] By way of example, the present application will be based mainlyon the MPEG type coding methods. Although the invention is not limitedto these standards, in the subsequent description reference will mainlybe made thereto.

[0007] The principle of such compression is recalled herein below.

[0008] In the video MPEG standards, compression of the video digitalsignals is obtained by utilizing the spatial redundancy and the temporalredundancy of the coded images.

[0009] The spatial redundancy is evaluated mainly by virtue of thesuccession of three operations: an operation commonly called theDiscrete Cosine Transform and denoted DCT, an operation of quantizingthe coefficients emanating from the DCT and a variable-length codingoperation for describing the quantized coefficients emanating from theDCT.

[0010] The temporal redundancy is analysed by a motion compensationoperation which consists, by translation of each block of the currentimage, in searching for the most similar block situated in a referenceimage. The analysis of the temporal redundancy leads to thedetermination of a field of translation vectors, which are commonlycalled motion vectors, as well as of a prediction error which is thedifference between the signal of the current image and the signal of theimage predicted by motion compensation. The prediction error is thenanalysed according to the principle of spatial redundancy.

[0011] MPEG coding is of predictive type. It follows that the decodingassociated therewith must be regularly reinitialised so as to protectthe signal against any transmission error or any signal break due to thetoggling of the decoder from one program to another.

[0012] To this end, the MPEG standard provides that, periodically, theimages have to be coded in spatial mode, that is to say according to amode utilizing only spatial redundancy. The images coded in spatial modeare called INTRA images or I images.

[0013] The images coded by utilizing temporal redundancy are of twotypes: on the one hand, the images constructed by reference to atemporally earlier image on the basis of a forward prediction and, onthe other hand, the images constructed by reference to two images,temporally earlier and later, on the basis of a forward prediction andof a backward prediction.

[0014] The coded images constructed on the basis of a forward predictionare called predicted images or P images and the coded images constructedon the basis of a forward prediction and of a backward prediction arecalled bi-directional images or B images.

[0015] An I image is decoded without making reference to images otherthan itself. A P image is decoded by making reference to the P or Iimage which precedes it. A B image is decoded by making reference to theI or P image which precedes it and to the I or P image which follows it.

[0016] The periodicity of the I images defines a group of imagescommonly denoted GOP (standing for Group Of Pictures).

[0017] Within one and the same GOP, the amount of data contained in an Iimage is generally greater than the amount of data contained in a Pimage and the amount of data contained in a P image is generally greaterthan the amount of data contained in a B image.

[0018] The person skilled in the art may refer to the ISO/IEC 11172 andISO/IEC 13818 standards for the details of implementation of this typeof coding. The degradation of the image brought about by the coding isnot perceptible to the eye of a viewer. However, when an image sequencehas undergone several decoding/coding cycles, the accumulation ofinformation losses is such that the degradation becomes perceptible.

[0019] Televisual broadcasting requires the effecting of several codingcycles. Specifically, the studio editing work for a film or for a pieceof reporting makes it necessary to code and to decode the image sequenceto be coded many times, conventionally between 5 and 10 times for apiece of reporting.

[0020] Now, a problem, arising during the cascading of several decodingand coding series, is the accumulation of information losses. After tendecoding/coding cycles, the image is of mediocre quality.

[0021] To remedy this problem, it is known to use recording help data.The person skilled in the art may for example refer to the internationalpatent applications published under the numbers WO 95/35628, WO 96/24222and WO 98/03017 or to European patent application EP-A-0 618 734. Theprinciple of recoding help data is the associating with each decodedimage of information relating to coding parameters used during theprevious coding so as to reuse these same parameters during the nextcoding of the image. Thus, the image information loss is limited afterthe first coding, thus permitting a considerable number ofdecoding/coding cycles while retaining good image quality.

[0022] Various possibilities of implementation are possible, some ofthem already being included in the SMPTE 319M, SMPTE 327M and SMPTE 329Mstandards. As far as MPEG coding is concerned, it is possible to reusevery many parameters. The main parameters reused are the image typewhich indicates whether the image undergoes intra-image coding,predictive coding or bidirectional predictive coding, and also thecoding mode which indicates whether the image is coded per frame or perfield. Commonly used complementary parameters are the resolution of theimage (or level), the distribution of the chrominance information (orprofile), the motion vectors, the quantization step sizes and thequantization matrices. Of course, all the coding parameters used (thatis to say all the parameters explicitly coded in the stream) are able tobe reused.

[0023] The recoding help data can be implemented in various ways. Toassociate the recoding help data with each image, it is known to use aschannel an auxiliary sound pathway, non-significant grey levelquantization bits, or the frame and/or line blanking intervals morecommonly known by the acronyms VBI and HBI (standing for VerticalBlanking Interval and Horizontal Blanking Interval).

[0024] The use of help data nevertheless has a few drawbacks. When theimage stream is modified, the help data are not modified accordingly.According to the technique of implementation, the recoding help data areeither deleted or retained according to the processing performed. If theimplementation systematically erases the help data, it becomesimpossible to reuse the information although some of it is usable. Byway of example, the image type and the coding mode are items ofinformation which can be retained despite the inlaying of a logo. If theimplementation does not erase the help data, the latter can in certaincases create more considerable losses of information than if they wereabsent, the use of erroneous motion vectors increasing the losses whenthe image stream is strongly compressed.

[0025] Other problems may arise when reusing help data envisaged forcertain coding conditions in a completely different context. MPEG codingoffers a range of use which varies from a few hundred Kb/s to 50 Mb/s.In order for the recoding to be able to benefit from the data to themaximum, the discrepancy between the recoding bit rate and the bit rateof the initial coding should not be too considerable. The use of certaincoding parameters corresponding to a high bit rate, for example 20 Mb/s,makes coding at low bit rate, for example 2 Mb/s, impossible.

[0026] The aim of the invention is to afford a solution for reusing thehelp data selectively. For this purpose, the invention will adjoin, intothe coded stream, at least one coding signature relating to the initialcoding. The coding signature is included in the coded stream so as to beable to preserve a trace of information related to the first coding,this information not being contained explicitly in the coded streamrepresentative of the images.

[0027] The subject of the invention is a method of coding a video imagestream with a view to a subsequent recoding using a same type of code,the code being an image compression code with information loss whichuses coding parameters, the said method providing, in a coded stream,coding parameters allowing the reconstruction of the video image stream,in which a coding signature is introduced into the coded stream and saidsignature is identified as a data item to be retained with a view to asubsequent recoding.

[0028] Preferably, the coding signature is representative of the codingconstraints. The expression coding constraints should be understood tomean the parameters used to specify the extreme limits of coding.

[0029] The subject of the invention is also a method of decoding a codedvideo image stream including a coding signature with a view to asubsequent recoding using a same type of code, the code being an imagecompression code with information loss which uses coding parameters, thesaid method including in the decoded signal some of the codingparameters which are used during the previous coding with a view to asubsequent reuse of the said parameters. During decoding, the codingsignature is incorporated into the decoded signal among the some of thecoding parameters.

[0030] In order to correct the problems related to modifications of thestream after decoding, the invention would adjoin information relatingto the integrity of the image. A decoding signature representative ofthe decoded image or of a zone of the decoded image is associated. Thedecoding signature is then placed in the decoded signal with thedecoding parameters. Thus, for each image or image zone, the coder iscapable of determining which information is still valid as a function ofthe decoding signature of the image. The introduction of a decodingsignature representative of an image zone within the help data makes itpossible to identify whether the zone is trustworthy and hence whetherthe help data are or are not valid.

[0031] According to a particular embodiment, the decoding signature is aword of n bits corresponding to a modular reduction of the sum of allthe bits of the luminance and chrominance levels of the image zone. Thistype of signature is well known in the area of transmissions for havinga signature representative of the integrity of a transmitted data packetand is better known by the name CRC (standing for Cyclic RedundancyCheck).

[0032] Preferably, the image is divided into a plurality of image zonesin such a way that each point of the image belongs to at least one zone,each zone having a decoding signature which is associated therewith. Thedividing of the image into several zones makes it possible to have moreaccuracy as regards the modification of the associated image.

[0033] The invention also relates to a method of processing a videoimage stream obtained by the method of the invention, in which the imagestream is slightly modified. After processing, a new calculation ofdecoding signature is performed so as to replace the decoding signaturesassociated with each image zone. The effecting of a new calculation ofdecoding signature makes it possible to render a processing devicetransparent. Only devices which modify the image very little should berendered transparent. According to a particular embodiment, theprocessing rendered transparent is a sequence which comprises arecording of the image stream on magnetic medium followed by a readingof said stream from said magnetic medium.

[0034] The invention also relates to a method of recoding a video imagestream previously decoded according to the decoding method which is thesubject of the invention, in which the recoding is carried out with theaid of a compression code of the same type as the code used previously,and in which parameters of the previous coding are transmitted with theimage stream, and in which, before undertaking the recoding, the codingand/or decoding signatures associated with the images of the said streamare verified so as to validate or to invalidate all or some of theparameters according to the said signatures.

[0035] According to a particular embodiment of the recoding method, thecode effecting a coding per group of images so as to recode the imagesof a group of images on the basis of information intrinsic to the saidgroup, all the parameters of the said group are invalidated andrecalculated if the elements of the coding signature associated with theimage stream are incompatible with certain recoding parameters.

[0036] Preferably, the recoding method uses a code which effects acoding per group of images, the said code coding the images of a groupon the basis of information intrinsic to the said group, in which firstcoding parameters are specific to the group of images or to each imageand second parameters are specific to image blocks of smaller size thanan image. All the second parameters of this zone or dependent on thiszone are invalidated and recalculated if a decoding signature associatedwith an image zone is not valid.

[0037] The invention will be better understood and other features andadvantages will become apparent on reading the description whichfollows, the description making reference to the appended drawings amongwhich:

[0038]FIG. 1 represents a transmission chain according to the state ofthe art,

[0039]FIG. 2 represents a transmission chain according to the invention,

[0040] FIGS. 3 to 5 represent constituent elements of the transmissionchain of FIG. 2,

[0041] FIGS. 6 to 8 represent examples of a transmission chain accordingto another aspect of the invention,

[0042]FIG. 9 represents an exemplary distribution of the image intoimage zones,

[0043]FIG. 10 represents an exemplary decoding signature generatingcircuit according to the invention,

[0044]FIG. 11 represents an exemplary decoding signature verificationcircuit according to the invention, and

[0045]FIG. 12 represents the transmission format of an uncompressedimage.

[0046]FIG. 1 represents an image transmission chain according to thestate of the art. An image sequence 1 is digitised then sent to a firstcoder 2, for example an MPEG type coder, so as to be compressed therein.The data representative of the image sequence 1 are then transmitted viaa first transmission channel 3. A decoder 4 receives the dataoriginating from the transmission channel 3 so as to decode them.

[0047] The decoder 4 is, for example, an MPEG type decoder whichsupplies on the one hand an image signal and on the other hand helpdata. With an explanatory aim, the image signal and the help data followseparate paths but can use the same physical transmission medium.Conventionally, the path used by the images is a high bit rate seriallink where the images are rated at a speed permitting direct display,when the processing is carried out digitally.

[0048] A second coder 5 receives the image signal and the help data soas to perform a new coding in order to send the image sequence over asecond transmission channel 6. As is known to the person skilled in theart, the help data serve to reuse the coding parameters used by thefirst coder 2 in the second coder 5. The first and second coders 2 and 5are subject respectively to first constraints Constraints1 and to secondconstraints Constraints2 which specify the coding conditions (bit rate,type of bit rate, limitation of GOP structure, etc.) of each coder. Asexplained previously, there may be incompatibilities between the firstconstraints Constraints1 and the second constraints Constraints2.

[0049]FIG. 2 represents a modified transmission chain according to theinvention. The first and second coders 2 and 5 are replaced with thirdand fourth coders 12 and 15. The decoder 4 is replaced with a decoder14. A coding signature representative of the first constraintsConstraints1 is sent through the channel 3 by the third coder 12. Thedecoder 14 identifies the coding signature and transmits it to thefourth coder 15 so that the latter can take account thereof during therecoding. The image signal, the help data and the coding signaturefollow three separate paths but may, obviously, use the same physicaltransmission medium.

[0050]FIG. 3 represents the design of the third coder 12. The thirdcoder 12 is, for example, an MPEG type coder which comprises at leastone audio encoder circuit 100, at least one video encoder circuit 101,at least one loss less compression circuit 102, and at least onepacketizing and multiplexing circuit 103, the said circuits 100 to 103being designed and devised according to known techniques of the MPEGstandard. In this figure, the sound signal has been parted from theimage signal since the MPEG standard makes provision to code themseparately; however, for the figures in which the sound signal does notappear, the latter is regarded as included in the image signal.

[0051] As known from the state of the art, MPEG coding defines a codingtechnique permitting numerous possibilities of implementation. The firstconstraints Constraints1 define the limit within which the coding mustbe performed and are supplied for this purpose to the encoder circuits100 and 101. The first constraints comprise one or more of the followingparameters:

[0052] nature of the bit rate of the coded stream, that is to saywhether it is a fixed or variable bit rate, this parameter depends onthe conditions of use of the channel 3,

[0053] value of the nominal or maximum stream, that is to say the bitrate requested of the coder, this parameter depends on the bandwidth ofthe channel 3,

[0054] type of image group structure, whether it is a fixed or variablestructure,

[0055] nominal (for a fixed GOP structure) or maximum (for a variableGOP structure) number of images in an image group,

[0056] nominal or maximum number of predicted images in an image group,

[0057] maximum number of successive images predicted in a bi-directionalmanner in an image group, this parameter depends on the capabilities ofthe coder 12,

[0058] maximum number of bits per group of images,

[0059] conformity to a particular format, the particular format beingdefinable for example in a standard so as to define one or more of theprevious parameters, and optionally other parameters, with the help of asingle word.

[0060] The constraint parameters are fixed by an operator as a functionof the hardware environment of which he is knowledgeable and of the usewhich will normally be made of the coded stream. The problem with theseparameters is that they are fixed at a given instant in a given contextand that the parameters in question do not necessarily correspond to theuse which will actually be made of the coded stream.

[0061] According to the invention, a signature computation circuit 104receives the various coding constraints Constraints1 corresponding to animage signal so as to carry out a shaping according to a predefinedformat, the shaped constraint parameters corresponding to the codingsignature. In order to make best use of the coding signature, theshaping should be carried out according to a standard recognized by allcoder manufacturers. The coding signature computation circuit willsupply the signature regularly to a loss less compression circuit 102,so that the coding signature is processed as user data. Regularly shouldbe understood to mean at least once per GOP.

[0062]FIG. 4 diagrammatically represents the design of the decoder 14.The decoder 14 is, for example, an MPEG type decoder which comprises atleast one audio decoder circuit 110, at least one video decoder circuit111, at least one loss less decompression circuit 112, and at least onecircuit for demultiplexing and separating the packets 113, the saidcircuits 110 to 113 being designed and devised according to knowntechniques of the MPEG standard. The person skilled in the art may notethat the decoder 113 corresponds to a conventional decoder on whichdigital data which correspond to the coding signature are recovered.With the aim of clarity, the coding signature is represented on a mediumindependent of the other signals. As is known to the person skilled inthe art, these coding signature digital data may be multiplexed withother digital data, for example the help data.

[0063]FIG. 5 represents the design of the fourth coder 15. The fourthcoder 15 comprises circuits identical to the third coder 12: at leastone audio encoder circuit 100, at least one video encoder circuit 101,at least one loss less compression circuit 102, at least one packetizingand multiplexing circuit 103, and a signature computation circuit 104.The coder 15 furthermore comprises a circuit for verifying theconstraints 105 and a circuit for validating the help data 106.

[0064] The circuit for validating the constraints 105 receives on theone hand the second constraints Constraints2 and on the other hand thecoding signature representative of the first constraints Constraints1.The circuit 105 performs a comparison of the first and second codingconstraints so as to determine whether the help data are or are notusable. The comparison is not made identically. By way of example, if astream reduction is performed with a factor of 3 or greater, the helpdata are generally no longer suited to the recoding constraint and theiruse renders the recoding of low performance or even impossible. Otherparameters may also indicate a coding incompatibility, if the maximumnumber of successive B images of the first constraints is greater thanthe maximum number of successive B images that can be managed by thecoder 15, the help data cannot be reused. As far as the nature of thestream is concerned, the help data can be reused if the change of bitrate so permits. The type of structure as well as the number of imagesper GOP may differ within certain limits if the bit rate ratio sopermits.

[0065] As a function of the comparison of the constraints, the circuitfor verifying the constraints 105 will supply a help data validationinformation item. By way of example, the validation information item maybe of binary type to validate or cancel all the help data. According toanother example, the cancellation of the help data may be moreselective, by giving the possibility of retaining the I, P or B type ofeach image.

[0066] The circuit for verifying the constraints supplies codingconstraints information which corresponds either to the secondconstraints, or to the first constraints, or else to a mixture of thefirst and second constraints. The coding constraints information will beused for the coding and sent to the signature computation circuit 104which will compute a coding signature representative of the constraintsof the coding effected.

[0067] A second aspect of the invention, relating to a decodingsignature, will now be described. The principle of the decodingsignature is to verify the integrity of the image before reusing thehelp data pertaining thereto.

[0068]FIG. 6 represents a part of an image transmission chain. Thedecoder 14 receives the data originating from the transmission channel 3so as to decode them. Preferably, the decoder 14 is an MPEG type decoderas described in FIG. 4. To simplify the drawing, the help data and thecoding signature (which are also denoted DA+SC) are represented on oneand the same path. Conventionally, the images and the help data aresynchronized. A fourth coder 15 receives the image signal and the helpdata so as to perform a new coding in order to send the image sequenceover a second transmission channel 6, as explained previously.

[0069] In order to be able to verify the integrity of the image, adecoding signature generating circuit 7 has been added at the output ofthe decoder 14. The generating circuit supplies a decoding signaturerepresentative of the information contained in the image. A decodingsignature verification circuit 8 receives the image signal and thesignature signal and supplies the fourth coder 15 with a signaturevalidity information item VALID so that the former can take account ofor ignore the help data.

[0070] The person skilled in the art will understand that the benefit ofsuch an invention arises when an item of equipment 9 or 10 is insertedbetween the decoder 14 and the fourth coder 15, as shown in FIGS. 7 and8. By way of example, the item of equipment 9 of FIG. 7 may be a videomixing table. According to the operation carried out, the output imagesignal may or may not be identical to the input image signal. Theverification circuit 8 will enable the fourth coder 15 to identify themodified images or image zones. As a function of the modifications made,the coder will therefore be able to reuse all or some of the help data.

[0071] The case of a mixing table corresponds to a specific case where apiece of reporting is edited in the studio and for which much of theimage sequences remains unchanged. On the other hand, when the image istouched up, the verification circuit indicates that a zone of the imagehas changed and only some of the help data is reusable.

[0072]FIG. 8 corresponds to the use of an item of equipment 10 which maybe regarded as transparent at coding level. As item of transparentequipment should be regarded devices which act on the image in a veryweak manner and which do not have the aim of modifying the image. By wayof example, the item of equipment 10 is a studio storage device, forexample a recorder in the DVCPRO format. The studio storage devicesusing tapes generally carry out a compression of the digital signal witha very slight loss which can be neglected relative to the lossesengendered by the MPEG-like transmission coding.

[0073] In order to render the item of equipment 10 completelytransparent, an additional generating circuit 7 b should be added whichrecalculates the decoding signatures from the image signal output by theitem of equipment 10. Thus, the few slightly modified pixels in an imagedo not invalidate the decoding signatures.

[0074] According to a preferred variant, the item of equipment 10 usesvideo image compression also according to the MPEG2 standard. The itemof equipment 10 can then verify the coding signature SC emanating fromthe previous coding and verifies that the losses which it introduces areactually negligible relative to the quality of the image resulting fromthe previous coding.

[0075] In the preferred example, the generating circuit 7 chops theimage into twelve image zones Z1 to Z12, as is represented in FIG. 9. Ifa decoding signature relating to one of the zones is invalidated, thenthe coder deactivates the help data relating to the said zone. By way ofexample for MPEG coding, the coder will ignore all the motion vectorscorresponding to the macroblocks of the said zone as well as all themotion vectors of the past or future images which make reference to thesaid zone.

[0076] If more than half of the decoding signatures of an image areinvalidated, then the coder also deactivates the parameters specific tothe image. In MPEG coding, the quantization step sizes and the DCTweighting matrices may be parameters specific to each image and hence beinvalidated if the number of modified zones is considerable.

[0077] As far as the parameters specific to the image sequence or to thegroup of images are concerned, these parameters should be retained evenif the images change. Specifically, the (I, P and B) image typeinformation may be retained despite the modifications since the use of aB image as I image is a potential source of loss of image information.

[0078]FIG. 10 represents a preferred embodiment of the generatingcircuit 7. The generating circuit 7 comprises four calculation circuits20 each having an input and an output, the inputs being linked togetherand receiving the image signal. Each of the calculation circuits 20 is aknown type of control key calculation circuit which performs a modularreduction of the data entering the said circuits. The calculationcircuits 20 are activated only when the image data present in the imagesignal correspond to the zone whose decoding signature they perform.When all the image data present in the various lines of a zone have beenentered into a calculation circuit 20, the said circuit 20 outputs thecorresponding decoding signature in series. A multiplexer 21 selects oneof the calculation circuits so as to multiplex the decoding signatureson the signature signal.

[0079]FIG. 11 represents a preferred embodiment of the signatureverification circuit 8. The verification circuit 8 comprises acalculation structure comprising four calculation circuits 22 and amultiplexer 23 which undertakes a function identical to the function ofthe generating circuit 7. A gate 24 of Exclusive-OR type with two inputsreceives on the one hand the signal output by the multiplexer 223 and,on the other hand, the signature signal originating from a generatingcircuit 7 so as to carry out a comparison between the signals and tosupply a VALID signal representative of the integrity of the imagezones, the VALID signal being active when a difference appears betweenthe transmitted decoding signature and the recalculated decodingsignature.

[0080] A great many variants of the invention are possible. According toone variant, the coding and decoding signatures are included in theimage signal. FIG. 12 represents an image transmitted according to aformat permitting direct display. As is known by the person skilled inthe art, the image transmitted comprises an actual image part and one ormore image parts VBI and HBI added for synchronisation. The added imageparts VBI and HBI serve to allow a cathode ray tube spot time to returnto the origin of a line (horizontal interval or HBI) or to the origin ofthe screen (vertical interval or VBI). The coding and decodingsignatures are included in the added intervals of the image so that thedecoding signatures remain synchronous with the image.

[0081] Another variant consists in including the coding and decodingsignature generation and verification circuits in the decoding andcoding circuits respectively. At the level of the structure, thearchitecture of the coding and decoding circuits comprising a pluralityof microprocessors coupled with memories, it is sufficient to modify theprogram installed in the circuits in order to include the coding anddecoding signature generating circuit or signature verification circuitfunction. Moreover, the horizontal interval and the vertical interval ofthe image are already used for the help data and make it possible toinclude the signatures signal in the said intervals.

1. Method of coding a video image stream with a view to a subsequentrecoding using a same type of code, the code being an image compressioncode with information loss which uses coding parameters, the said methodproviding, in a coded stream, coding parameters allowing thereconstruction of the video image stream, characterized in that a codingsignature is introduced into the coded stream and said signature isidentified as a data item to be retained with a view to a subsequentrecoding.
 2. Method according to claim 1, characterized in that thecoding signature is representative of the coding constraints.
 3. Methodaccording to claim 2, characterized in that the coding constraintscomprise one or more of the following parameters: nature of the bit rateof the coded stream, value of the nominal or maximum stream, type ofimage group structure, nominal or maximum number of images in an imagegroup, nominal or maximum number of predicted images in an image group,maximum number of successive images predicted in a bi-directional mannerin an image group, maximum number of bits per image group, conformity toa particular format.
 4. Method of decoding a video image stream codedaccording to the method of one of claims 1 to 3 with a view to asubsequent recoding using a same type of code, the code being an imagecompression code with information loss which uses coding parameters, thesaid method including in the decoded signal some of the codingparameters which are used during the previous coding with a view to asubsequent reuse of the said parameters, characterized in that, duringdecoding, the coding signature is incorporated into the decoded signalamong the some of the coding parameters.
 5. Method according to claim 4,characterized in that, during decoding, at least one decoding signaturerepresentative of at least one zone of each decoded image of the streamis associated and this signature is placed in the decoded signal withthe coding parameters.
 6. Method according to claim 5, characterized inthat the decoding signature is a word of n bits corresponding to amodular reduction of the sum of all the bits of the luminance andchrominance levels of the image zone.
 7. Method according to one ofclaims 5 or 6, characterized in that the image is divided into aplurality of image zones in such a way that each point of the imagebelongs to at least one zone, each zone having a decoding signaturewhich is associated therewith.
 8. Method of processing a video imagestream obtained by the method of one of claims 5 to 7, in which theimage stream is slightly modified, characterized in that, afterprocessing, a new calculation of decoding signature is performed so asto replace the signatures associated with each image zone.
 9. Methodaccording to claim 8, characterized in that the processing is a sequencewhich comprises a recording of the image stream on magnetic mediumfollowed by a reading of said stream from said magnetic medium. 10.Method of recoding a video image stream previously decoded according tothe method of one of claims 4 to 9, in which the recoding is carried outwith the aid of a compression code of the same type as the code usedpreviously, and in which parameters of the previous coding aretransmitted with the image stream, characterized in that, beforeundertaking the recoding, the coding and/or decoding signaturesassociated with the images of the said stream are verified so as tovalidate or to invalidate all or some of the parameters according to thesaid signatures.
 11. Method according to claim 10, in which the codeeffects a coding per group of images, the said code recoding the imagesof a group of images on the basis of information intrinsic to the saidgroup, characterized in that, if the elements of the coding signatureassociated with the image stream are incompatible with recodingparameters, then all the parameters of the said group are invalidatedand recalculated.
 12. Method according to claim 10, in which the codeeffects a coding per group of images, the said code coding the images ofa group on the basis of information intrinsic to the said group, inwhich first coding parameters are specific to the group of images or toeach image and second parameters are specific to image blocks of smallersize than an image, characterized in that if a decoding signatureassociated with an image zone is not valid, then all the secondparameters of this zone or dependent on this zone are invalidated andrecalculated.